1. Field of the Invention
The present invention relates to a method of testing the quality of circuits interconnecting terminals and a network and the action of the network by using channels B1 and B2 of a frame specified in CCITT-I. 430.
2. Description of the Prior Art
The connection of a network and terminals will be described with reference to FIG. 7.
Shown in FIG. 7 are terminals 11, 12 and 13, a network (hereinafter, abbreviated to "NW") 23, and a digital service unit (hereinafter, abbreviated to "DSU") 24. The NW 23 is the general designation of a system including the DSU 24, transmission lines, switching system, and the associated equipment. The teminals 11, 12 and 13 of the ISDN are connected to the DSU 24 by a T-line 21 and an R-line 22 for communication with terminals connected to the NW 23. The T-line 21 and the R-line 22 constitute a data bus. According to CCITT-I. 430, eight terminals at the maximum can be connected to a DSU.
FIG. 8 shows an essential portion of a diagram 3/I. 430 of a frame system specified by CCITT-I. 430. As shown in FIG. 8, a frame system comprises a frame A for transmitting data from the terminal 11 to the DSU 24, and a frame B for transmitting data from the DSU 24 to the terminal 11. The frames A and B for exchanging data between the terminals and the DSU 24 have channels D for signals, and channels B1 and B2 for data. The-frame B has channels E designated as echo bits. Since eight terminals at the maximum are connected to a single DSU, race in transmission from the terminals to the DSU along the channel D must be controlled. The channels E are used for race control. The terminal must carry out a channel D call control procedure along the channel D to acquire the right of use of the channel B. Upon the acquisition of the right of use of a line, the terminal sends necessary information along the channel B of the frame A to a called terminal. However, the conventional terminal equipment is unable to decide whether or not the information sent out is received correctly through the NW 23 by the called terminal.